In Py3k:

$ ./python Lib/test/regrtest.py test_decimal test_builtin
test_decimal
test_builtin
test test_builtin failed -- Traceback (most recent call last):
  File "Lib/test/regrtest.py", line 603, in runtest_inner
    indirect_test()
  File "/temp/python/py3k/Lib/test/test_builtin.py", line 1252, in test_main
    run_unittest(*test_classes)
  File "/temp/python/py3k/Lib/test/support.py", line 717, in run_unittest
    _run_suite(suite)
  File "/temp/python/py3k/Lib/test/support.py", line 700, in _run_suite
    raise TestFailed(err)
test.support.TestFailed: Traceback (most recent call last):
  File "/temp/python/py3k/Lib/test/test_builtin.py", line 390, in
test_general_eval
    self.assertEqual(eval('globals()', g, m), g)
AssertionError: {'unittest': <module 'unittest' from
'/temp/python/py3k/Lib/unittest.py'>, 'random': <module 'random' from
'/temp/python/py3k/Lib/random.py'>, 'test_conv_no_sign': [('0', 0),
('1', 1), ('9', 9), ('10', 10), ('99', 99), ('100', 100), ('314', 314),
(' 314', 314), ('314 ', 314), ('  \t\t  314  \t\t  ', 314),
('2147483647', 2147483647), ('  1x', <class 'ValueError'>), ('  1  ',
1), ('  1\x02  ', <class 'ValueError'>), ('', <class 'ValueError'>), ('
', <class 'ValueError'>), ('  \t\t  ', <class 'ValueError'>),
('\u0663\u0661\u0664 ', 314), ('\u0200', <class 'ValueError'>)],
'test_conv_sign': [('0', 0), ('1', 1), ('9', 9), ('10', 10), ('99', 99),
('100', 100), ('314', 314), (' 314', <class 'ValueError'>), ('314 ',
314), ('  \t\t  314  \t\t  ', <class 'ValueError'>), ('2147483647',
2147483647), ('  1x', <class 'ValueError'>), ('  1  ', <class
'ValueError'>), ('  1\x02  ', <class 'ValueError'>), ('', <class
'ValueError'>), (' ', <class 'ValueError'>), ('  \t\t  ', <class
'ValueError'>), ('\u0663\u0661\u0664 ', 314), ('\u0200', <class
'ValueError'>)], 'run_with_locale': <function run_with_locale at
0x10b0390>, 'io': <module 'io' from '/temp/python/py3k/Lib/io.py'>,
'Squares': <class 'test.test_builtin.Squares'>, 'neg': <built-in
function neg>, '__package__': None, 'collections': <module 'collections'
from '/temp/python/py3k/Lib/collections.py'>, 'TestFailingIter': <class
'test.test_builtin.TestFailingIter'>, 'run_unittest': <function
run_unittest at 0x10b0540>, '__doc__': None, 'test_main': <function
test_main at 0x1474390>, 'fractions': <module 'fractions' from
'/temp/python/py3k/Lib/fractions.py'>, 'StrSquares': <class
'test.test_builtin.StrSquares'>, 'warnings': <module 'warnings' from
'/temp/python/py3k/Lib/warnings.py'>, '__builtins__': {'bytearray':
<class 'bytearray'>, 'IndexError': <class 'IndexError'>, 'all':
<built-in function all>, 'help': Type help() for interactive help, or
help(object) for help about object., 'vars': <built-in function vars>,
'SyntaxError': <class 'SyntaxError'>, 'UnicodeDecodeError': <class
'UnicodeDecodeError'>, 'memoryview': <class 'memoryview'>, 'isinstance':
<built-in function isinstance>, '__build_class__': <built-in function
__build_class__>, 'copyright': Copyright (c) 2001-2008 Python Software
Foundation.
All Rights Reserved.

Copyright (c) 2000 BeOpen.com.
All Rights Reserved.

Copyright (c) 1995-2001 Corporation for National Research Initiatives.
All Rights Reserved.

Copyright (c) 1991-1995 Stichting Mathematisch Centrum, Amsterdam.
All Rights Reserved., 'NameError': <class 'NameError'>, 'BytesWarning':
<class 'BytesWarning'>, 'dict': <class 'dict'>, 'IOError': <class
'IOError'>, 'oct': <built-in function oct>, 'bin': <built-in function
bin>, 'SystemExit': <class 'SystemExit'>, 'format': <built-in function
format>, 'repr': <built-in function repr>, 'sorted': <built-in function
sorted>, 'False': False, 'RuntimeWarning': <class 'RuntimeWarning'>,
'list': <class 'list'>, 'iter': <built-in function iter>, 'Warning':
<class 'Warning'>, '__package__': None, 'round': <built-in function
round>, 'dir': <built-in function dir>, 'cmp': <built-in function cmp>,
'set': <class 'set'>, 'bytes': <class 'bytes'>, 'UnicodeTranslateError':
<class 'UnicodeTranslateError'>, 'issubclass': <built-in function
issubclass>, 'EOFError': <class 'EOFError'>, 'locals': <built-in
function locals>, 'BufferError': <class 'BufferError'>, 'slice': <class
'slice'>, 'FloatingPointError': <class 'FloatingPointError'>, 'sum':
<built-in function sum>, 'getattr': <built-in function getattr>, 'abs':
<built-in function abs>, 'exit': Use exit() or Ctrl-D (i.e. EOF) to
exit, 'print': <built-in function print>, 'True': True, 'FutureWarning':
<class 'FutureWarning'>, 'ImportWarning': <class 'ImportWarning'>,
'None': None, 'hash': <built-in function hash>, 'ReferenceError': <class
'ReferenceError'>, 'len': <built-in function len>, 'credits':     Thanks
to CWI, CNRI, BeOpen.com, Zope Corporation and a cast of thousands
    for supporting Python development.  See www.python.org for more
information., 'frozenset': <class 'frozenset'>, '__name__': 'builtins',
'ord': <built-in function ord>, 'super': <class 'super'>, '_':
Decimal('NaN123'), 'TypeError': <class 'TypeError'>, 'license': Type
license() to see the full license text, 'KeyboardInterrupt': <class
'KeyboardInterrupt'>, 'UserWarning': <class 'UserWarning'>, 'filter':
<class 'filter'>, 'range': <class 'range'>, 'staticmethod': <class
'staticmethod'>, 'SystemError': <class 'SystemError'>, 'BaseException':
<class 'BaseException'>, 'pow': <built-in function pow>, 'RuntimeError':
<class 'RuntimeError'>, 'float': <class 'float'>, 'MemoryError': <class
'MemoryError'>, 'StopIteration': <class 'StopIteration'>, 'globals':
<built-in function globals>, 'divmod': <built-in function divmod>,
'enumerate': <class 'enumerate'>, 'Ellipsis': Ellipsis, 'LookupError':
<class 'LookupError'>, 'open': <class 'io.OpenWrapper'>, 'quit': Use
quit() or Ctrl-D (i.e. EOF) to exit, 'UnicodeError': <class
'UnicodeError'>, 'zip': <class 'zip'>, 'hex': <built-in function hex>,
'next': <built-in function next>, 'ImportError': <class 'ImportError'>,
'chr': <built-in function chr>, 'type': <class 'type'>, '__doc__':
"Built-in functions, exceptions, and other objects.\n\nNoteworthy: None
is the `nil' object; Ellipsis represents `...' in slices.", 'Exception':
<class 'Exception'>, 'tuple': <class 'tuple'>, 'reversed': <class
'reversed'>, 'UnicodeEncodeError': <class 'UnicodeEncodeError'>,
'input': <built-in function input>, 'hasattr': <built-in function
hasattr>, 'delattr': <built-in function delattr>, 'setattr': <built-in
function setattr>, 'SyntaxWarning': <class 'SyntaxWarning'>, 'compile':
<built-in function compile>, 'ArithmeticError': <class
'ArithmeticError'>, 'str': <class 'str'>, 'property': <class
'property'>, 'GeneratorExit': <class 'GeneratorExit'>, 'int': <class
'int'>, '__import__': <built-in function __import__>, 'KeyError': <class
'KeyError'>, 'PendingDeprecationWarning': <class
'PendingDeprecationWarning'>, 'EnvironmentError': <class
'EnvironmentError'>, 'ascii': <built-in function ascii>, 'id': <built-in
function id>, 'OSError': <class 'OSError'>, 'DeprecationWarning': <class
'DeprecationWarning'>, 'min': <built-in function min>, 'UnicodeWarning':
<class 'UnicodeWarning'>, 'any': <built-in function any>, 'complex':
<class 'complex'>, 'bool': <class 'bool'>, 'ValueError': <class
'ValueError'>, 'NotImplemented': NotImplemented, 'map': <class 'map'>,
'exec': <built-in function exec>, 'max': <built-in function max>,
'object': <class 'object'>, 'TabError': <class 'TabError'>,
'ZeroDivisionError': <class 'ZeroDivisionError'>, 'eval': <built-in
function eval>, '__debug__': True, 'IndentationError': <class
'IndentationError'>, 'AssertionError': <class 'AssertionError'>,
'classmethod': <class 'classmethod'>, 'UnboundLocalError': <class
'UnboundLocalError'>, 'NotImplementedError': <class
'NotImplementedError'>, 'AttributeError': <class 'AttributeError'>,
'OverflowError': <class 'OverflowError'>}, '__file__':
'/temp/python/py3k/Lib/test/test_builtin.py', 'BuiltinTest': <class
'test.test_builtin.BuiltinTest'>, 'TestFailingBool': <class
'test.test_builtin.TestFailingBool'>, 'sys': <module 'sys' (built-in)>,
'test': <module 'test' from '/temp/python/py3k/Lib/test/__init__.py'>,
'__name__': 'test.test_builtin', 'unlink': <function unlink at
0x7a5540>, 'TESTFN': '@test', 'TestSorted': <class
'test.test_builtin.TestSorted'>, 'BitBucket': <class
'test.test_builtin.BitBucket'>, 'fcmp': <function fcmp at 0x7ad540>} !=
{'unittest': <module 'unittest' from
'/temp/python/py3k/Lib/unittest.py'>, 'random': <module 'random' from
'/temp/python/py3k/Lib/random.py'>, 'test_conv_no_sign': [('0', 0),
('1', 1), ('9', 9), ('10', 10), ('99', 99), ('100', 100), ('314', 314),
(' 314', 314), ('314 ', 314), ('  \t\t  314  \t\t  ', 314),
('2147483647', 2147483647), ('  1x', <class 'ValueError'>), ('  1  ',
1), ('  1\x02  ', <class 'ValueError'>), ('', <class 'ValueError'>), ('
', <class 'ValueError'>), ('  \t\t  ', <class 'ValueError'>),
('\u0663\u0661\u0664 ', 314), ('\u0200', <class 'ValueError'>)],
'test_conv_sign': [('0', 0), ('1', 1), ('9', 9), ('10', 10), ('99', 99),
('100', 100), ('314', 314), (' 314', <class 'ValueError'>), ('314 ',
314), ('  \t\t  314  \t\t  ', <class 'ValueError'>), ('2147483647',
2147483647), ('  1x', <class 'ValueError'>), ('  1  ', <class
'ValueError'>), ('  1\x02  ', <class 'ValueError'>), ('', <class
'ValueError'>), (' ', <class 'ValueError'>), ('  \t\t  ', <class
'ValueError'>), ('\u0663\u0661\u0664 ', 314), ('\u0200', <class
'ValueError'>)], 'run_with_locale': <function run_with_locale at
0x10b0390>, 'io': <module 'io' from '/temp/python/py3k/Lib/io.py'>,
'Squares': <class 'test.test_builtin.Squares'>, 'neg': <built-in
function neg>, '__package__': None, 'collections': <module 'collections'
from '/temp/python/py3k/Lib/collections.py'>, 'TestFailingIter': <class
'test.test_builtin.TestFailingIter'>, 'run_unittest': <function
run_unittest at 0x10b0540>, '__doc__': None, 'test_main': <function
test_main at 0x1474390>, 'fractions': <module 'fractions' from
'/temp/python/py3k/Lib/fractions.py'>, 'StrSquares': <class
'test.test_builtin.StrSquares'>, 'warnings': <module 'warnings' from
'/temp/python/py3k/Lib/warnings.py'>, '__builtins__': {'bytearray':
<class 'bytearray'>, 'IndexError': <class 'IndexError'>, 'all':
<built-in function all>, 'help': Type help() for interactive help, or
help(object) for help about object., 'vars': <built-in function vars>,
'SyntaxError': <class 'SyntaxError'>, 'UnicodeDecodeError': <class
'UnicodeDecodeError'>, 'memoryview': <class 'memoryview'>, 'isinstance':
<built-in function isinstance>, '__build_class__': <built-in function
__build_class__>, 'copyright': Copyright (c) 2001-2008 Python Software
Foundation.
All Rights Reserved.

Copyright (c) 2000 BeOpen.com.
All Rights Reserved.

Copyright (c) 1995-2001 Corporation for National Research Initiatives.
All Rights Reserved.

Copyright (c) 1991-1995 Stichting Mathematisch Centrum, Amsterdam.
All Rights Reserved., 'NameError': <class 'NameError'>, 'BytesWarning':
<class 'BytesWarning'>, 'dict': <class 'dict'>, 'IOError': <class
'IOError'>, 'oct': <built-in function oct>, 'bin': <built-in function
bin>, 'SystemExit': <class 'SystemExit'>, 'format': <built-in function
format>, 'repr': <built-in function repr>, 'sorted': <built-in function
sorted>, 'False': False, 'RuntimeWarning': <class 'RuntimeWarning'>,
'list': <class 'list'>, 'iter': <built-in function iter>, 'Warning':
<class 'Warning'>, '__package__': None, 'round': <built-in function
round>, 'dir': <built-in function dir>, 'cmp': <built-in function cmp>,
'set': <class 'set'>, 'bytes': <class 'bytes'>, 'UnicodeTranslateError':
<class 'UnicodeTranslateError'>, 'issubclass': <built-in function
issubclass>, 'EOFError': <class 'EOFError'>, 'locals': <built-in
function locals>, 'BufferError': <class 'BufferError'>, 'slice': <class
'slice'>, 'FloatingPointError': <class 'FloatingPointError'>, 'sum':
<built-in function sum>, 'getattr': <built-in function getattr>, 'abs':
<built-in function abs>, 'exit': Use exit() or Ctrl-D (i.e. EOF) to
exit, 'print': <built-in function print>, 'True': True, 'FutureWarning':
<class 'FutureWarning'>, 'ImportWarning': <class 'ImportWarning'>,
'None': None, 'hash': <built-in function hash>, 'ReferenceError': <class
'ReferenceError'>, 'len': <built-in function len>, 'credits':     Thanks
to CWI, CNRI, BeOpen.com, Zope Corporation and a cast of thousands
    for supporting Python development.  See www.python.org for more
information., 'frozenset': <class 'frozenset'>, '__name__': 'builtins',
'ord': <built-in function ord>, 'super': <class 'super'>, '_':
Decimal('NaN123'), 'TypeError': <class 'TypeError'>, 'license': Type
license() to see the full license text, 'KeyboardInterrupt': <class
'KeyboardInterrupt'>, 'UserWarning': <class 'UserWarning'>, 'filter':
<class 'filter'>, 'range': <class 'range'>, 'staticmethod': <class
'staticmethod'>, 'SystemError': <class 'SystemError'>, 'BaseException':
<class 'BaseException'>, 'pow': <built-in function pow>, 'RuntimeError':
<class 'RuntimeError'>, 'float': <class 'float'>, 'MemoryError': <class
'MemoryError'>, 'StopIteration': <class 'StopIteration'>, 'globals':
<built-in function globals>, 'divmod': <built-in function divmod>,
'enumerate': <class 'enumerate'>, 'Ellipsis': Ellipsis, 'LookupError':
<class 'LookupError'>, 'open': <class 'io.OpenWrapper'>, 'quit': Use
quit() or Ctrl-D (i.e. EOF) to exit, 'UnicodeError': <class
'UnicodeError'>, 'zip': <class 'zip'>, 'hex': <built-in function hex>,
'next': <built-in function next>, 'ImportError': <class 'ImportError'>,
'chr': <built-in function chr>, 'type': <class 'type'>, '__doc__':
"Built-in functions, exceptions, and other objects.\n\nNoteworthy: None
is the `nil' object; Ellipsis represents `...' in slices.", 'Exception':
<class 'Exception'>, 'tuple': <class 'tuple'>, 'reversed': <class
'reversed'>, 'UnicodeEncodeError': <class 'UnicodeEncodeError'>,
'input': <built-in function input>, 'hasattr': <built-in function
hasattr>, 'delattr': <built-in function delattr>, 'setattr': <built-in
function setattr>, 'SyntaxWarning': <class 'SyntaxWarning'>, 'compile':
<built-in function compile>, 'ArithmeticError': <class
'ArithmeticError'>, 'str': <class 'str'>, 'property': <class
'property'>, 'GeneratorExit': <class 'GeneratorExit'>, 'int': <class
'int'>, '__import__': <built-in function __import__>, 'KeyError': <class
'KeyError'>, 'PendingDeprecationWarning': <class
'PendingDeprecationWarning'>, 'EnvironmentError': <class
'EnvironmentError'>, 'ascii': <built-in function ascii>, 'id': <built-in
function id>, 'OSError': <class 'OSError'>, 'DeprecationWarning': <class
'DeprecationWarning'>, 'min': <built-in function min>, 'UnicodeWarning':
<class 'UnicodeWarning'>, 'any': <built-in function any>, 'complex':
<class 'complex'>, 'bool': <class 'bool'>, 'ValueError': <class
'ValueError'>, 'NotImplemented': NotImplemented, 'map': <class 'map'>,
'exec': <built-in function exec>, 'max': <built-in function max>,
'object': <class 'object'>, 'TabError': <class 'TabError'>,
'ZeroDivisionError': <class 'ZeroDivisionError'>, 'eval': <built-in
function eval>, '__debug__': True, 'IndentationError': <class
'IndentationError'>, 'AssertionError': <class 'AssertionError'>,
'classmethod': <class 'classmethod'>, 'UnboundLocalError': <class
'UnboundLocalError'>, 'NotImplementedError': <class
'NotImplementedError'>, 'AttributeError': <class 'AttributeError'>,
'OverflowError': <class 'OverflowError'>}, '__file__':
'/temp/python/py3k/Lib/test/test_builtin.py', 'BuiltinTest': <class
'test.test_builtin.BuiltinTest'>, 'TestFailingBool': <class
'test.test_builtin.TestFailingBool'>, 'sys': <module 'sys' (built-in)>,
'test': <module 'test' from '/temp/python/py3k/Lib/test/__init__.py'>,
'__name__': 'test.test_builtin', 'unlink': <function unlink at
0x7a5540>, 'TESTFN': '@test', 'TestSorted': <class
'test.test_builtin.TestSorted'>, 'BitBucket': <class
'test.test_builtin.BitBucket'>, 'fcmp': <function fcmp at 0x7ad540>}


During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "Lib/test/regrtest.py", line 1197, in <module>
    main()
  File "Lib/test/regrtest.py", line 411, in main
    testdir, huntrleaks)
  File "Lib/test/regrtest.py", line 570, in runtest
    testdir, huntrleaks)
  File "Lib/test/regrtest.py", line 623, in runtest_inner
    print("test", test, "failed --", msg)
  File "/temp/python/py3k/Lib/io.py", line 1459, in write
    b = encoder.encode(s)
  File "/temp/python/py3k/Lib/encodings/ascii.py", line 22, in encode
    return codecs.ascii_encode(input, self.errors)[0]
UnicodeEncodeError: 'ascii' codec can't encode characters in position
697-699: ordinal not in range(128)

My wild uneducated guess is that it's due to a nan-like value being in
the globals, and comparing unequal to itself:

>>> d = {1: float('nan')}
>>> d
{1: nan}
>>> d == d
False
>>> import decimal
>>> nan = decimal.Decimal('nan')
>>> d = {1: nan}
>>> d == d
False

Bingo! doctest sets the __builtins__._ variable,
and the last doctest in decimal.py (in lexicographic order) is
Decimal.__round__, which ends with
        >>> round(Decimal('sNaN123'), 0)
        Decimal('NaN123')
This is the value you see in Benjamin's output.

Here is a trivial patch:

Index: Lib/test/regrtest.py
===================================================================
--- Lib/test/regrtest.py        (revision 65283)
+++ Lib/test/regrtest.py        (working copy)
@@ -647,6 +647,8 @@
 def cleanup_test_droppings(testname, verbose):
     import shutil

+    __builtins__._ = None
+
     # Try to clean up junk commonly left behind.  While tests shouldn't
leave
     # any files or directories behind, when a test fails that can be
tedious
     # for it to arrange.  The consequences can be especially nasty on
Windows,

On Tue, Jul 29, 2008 at 8:30 AM, Amaury Forgeot d'Arc
<report@bugs.python.org> wrote:
>
> Amaury Forgeot d'Arc <amauryfa@gmail.com> added the comment:
>
> Bingo! doctest sets the __builtins__._ variable,
> and the last doctest in decimal.py (in lexicographic order) is
> Decimal.__round__, which ends with
>        >>> round(Decimal('sNaN123'), 0)
>        Decimal('NaN123')
> This is the value you see in Benjamin's output.
>
> Here is a trivial patch:

Shouldn't this be fixed in test_decimal so it cleans properly after it's self?

I think the cleanup should either be in regrtest.py as Amaury proposes,
or in doctest itself. There's nothing wrong in decimal and its test suite.

I was leaning towards the "doctest should not pollute the __builtins__
module" side. 
test_decimal might not be the only test that leaks one object outside
the module.

I vote for fixing doctest then!

Here is another patch that modifies doctest itself.
Note that the _ is cleared only if "clear_globs" is True, which makes
sense IMO.

Index: Lib/doctest.py
===================================================================
--- Lib/doctest.py      (revision 65283)
+++ Lib/doctest.py      (working copy)
@@ -1360,6 +1360,7 @@
             linecache.getlines = self.save_linecache_getlines
             if clear_globs:
                 test.globs.clear()
+                __builtins__['_'] = None

     #/////////////////////////////////////////////////////////////////
     # Summarization

The doctest patch looks fine to me.

Well except that it would be better spelt as:
    __builtins__._ = None

Ok. Thanks! Applied in r65285.

On Tue, Jul 29, 2008 at 10:29 AM, Antoine Pitrou <report@bugs.python.org> wrote:
>
> Antoine Pitrou <pitrou@free.fr> added the comment:
>
> Well except that it would be better spelt as:
>    __builtins__._ = None

__builtins__ is a dict here, though.
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <http://bugs.python.org/issue3462>
> _______________________________________
>

-- 
Cheers,
Benjamin Peterson
"There's no place like 127.0.0.1."

You know what? It's a mess.
- from the __main__ module, __builtins__ is a module.
- in all other modules, __builtins__ is a dict.

The fix is correct for most modules:
    ./python Lib\test\test_decimal.py
but fails for
    ./python Lib\doctest.py

Selon Amaury Forgeot d'Arc <report@bugs.python.org>:
>
> You know what? It's a mess.
> - from the __main__ module, __builtins__ is a module.
> - in all other modules, __builtins__ is a dict.

(why is it so? :-O)

Anyway:

    import builtins as b
    b._ = None

Ok, I've found it:

« But why is __builtins__ a module in __main__ and a dict elsewhere?
Because in *interactive* mode, printing vars() would include
__builtins__, which would be rather large. Code that incorrectly
assumes it's always one or the other is incorrect and has always been
incorrect; the wart was present when this feature first appeared.
Since this has never been documented AFAIK, it's probably just been
(incorrectly) reverse-engineered and copied around. »

http://mail.python.org/pipermail/python-3000/2007-March/006170.html

:-S

It is actually documented, at the bottom of
http://docs.python.org/dev/library/__builtin__.html

"import builtin" is the correct thing to do.